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Advise on improving attic ventilation

0 votes

Hi,

I bought my house two years ago and noticed severe ice dams each winter. After doing some research I realized that the attic is not properly ventilated (I can't do much about improving the insulation). There are only 3 roof vents which I estimate represent 1/446 of ventilated area relative to the total attic area (about 1200 sf). The house has soffits on all sides so I thought about installing soffit vents, but the shape of the roof is kind of unusual. The back of the roof is almost flat so I'm not sure if soffit vents will make a difference. Also the front of the roof has horizontal  eaves  only partially. The eaves on the sides are not horizontal either. I included a draft of the roof line below. So my question is, will soffit vents improve the ventilation for this kind of roof? If so, where they should be installed and how many (quantity/size)? My best guess is to install at least 3 in the back and 3 in the front with a size of 4"x16" each, to match the exhaust vents . In front, 2 vents would be intalled on the lower end of the sloped eaves. Would that work?

Thank you. 

 

 

 

asked in Attic Area by iromosan (130 points)
edited by iromosan
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1 Answer

+1 vote

 

Hi Iromosan,
 
Although you are in a cold winter weather zone and most likely there’s a vapor retarder beneath the attic’s floor insulation layer I’d go with a no-vapor retarder standard for the attic ventilation calculations. It’s always best to keep your unconditioned attic as cold as possible.
 
We’ll be using a formula based on 1 square foot of attic ventilation for every 150 square feet of attic floor space balanced equally between the soffit intake vents and exhaust vents along the ridge section of the roof. For your 1200 square foot attic space: 
 
1200 divided by 150 = 8 square feet of attic ventilation which needs to be evenly distributed between the upper and lower portion of the roof: 8 divided by 2 = 4 sq feet for intake and 4 sq feet for exhaust.
 
Since all the attic vents have net free air flow specification in inches we need to multiply our results by 144. 
4 sq feet x 144 = 576 square inches of attic ventilation for intake and 576 for exhaust. 
 
The next step is to decide which type of vent to use for the soffit; I would recommend continuous soffit vent because it delivers air flow to each and every single rafter bay. You can find specifications on attic ventilation products in your local home improvement store, as an example you can check the AirVent brand here - http://www.airvent.com/homeowner/products/intakeSoffit-specs.shtml
 
If there’s no labels on the existing roof vents, measure the opening under the vent and compare to a vent with similar dimensions in the store. Remember that the actual roof decking cutout, let’s say 10”x10” will not provide100” of net free area because of the screens and other obstruction that might be part of a vent design. A typical static roof vent (something like on your drawing) provides approximately 50 sq inches of net free area.
 
I marked your vent locations on the drawing below (just like you’ve mentioned except for the continuous recommendation instead of smaller ones), it should work. I just don’t know how much room there’s in your attic’s rear / almost flat section, hopefully there’s sufficient space between the insulation and the roof decking to provide air flow from the soffit.
 
Balancing attic air flow
 
Let me know if you have more questions, just click “comment” below
 
answered by darekrudy (21,730 points)

 

Thank you. I greatly appreciate your advice. 

You are welcome. I'm glad I can help.
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